1. Field of the Invention
The invention generally relates to recovery of catonic and anionic values from used or spent pickling solutions used in the treatment of products formed of certain metals and metal alloys. In particular, the invention concerns the recovery of zirconium from acidic pickling solutions, residual fluoride and nitrate ions also being recovered for recycle.
2. Description of the Prior Art
The conversion of ingots of zirconium and zirconium alloys to finished products necessitates a very high level of care to ensure that surface defects, which may possibly occur during each of the stages of the conversion operation, do not create problems during subsequent stages of said operation which could cause the finished products to be rejected. Such care is justified not only due to the high cost of the raw materials which must be subjected to expensive purification and reduction treatments, but also because of extremely strict specifications which must be set by the nuclear industry regarding materials, structural members and combustible elements, the zirconium products so treated being typically used as structural or sheathing materials for nuclear reactors. For all these reasons, pickling treatments are carried out on zirconium or zirconium alloy products at various stages in the production operations. Most generally, such pickling treatments are carried out in aqueous solutions which simultaneously contain nitric acid and hydrofluoric acid. The concentration of these acids varies but is generally of the order of from 2 to 5 N for nitric acid and from 0.5 to 2 N for hydrofluoric acid. Upon coming into contact with zirconium and its alloys, such pickling solutions gradually become charged with zirconium and various metals, mainly metals of addition, while their pH-values decrease. Experience has shown that the pickling speed decreases in proportion to an increase in the concentration in respect to dissolved zirconium and that, beyond a certain threshold, it is found that particles of solid phases appear, such particles having a tendency to precipitate. Introducing fresh amounts of HF and HNO.sub.3 only delays this precipitation phenomenon but does not suppress it.
In practice, it is necessary to remove the pickling solutions as soon as their dissolved zirconium content substantially exceeds 25 g/liter. Ths requirement results in the dumping of large amounts of the solutions, which solutions have a high fluoride and nitrate ion content, while also containing relatively substantial amounts of zirconium. Dumping these solutions represents substantial wastage of zirconium and acids and is also a cause of pollution. The amounts of zirconium or zirconium-based alloys which are wasted increase as the stage at which the pickling treatments are carried out approaches the final state in which the product is most frequently in the form of tubes or sheets of small thickness with a high surface-to-volume ratio. The loss of weight in the course of a single pickling operation can then reach 5% or more.
The process which is the subject of this invention makes it possible simultaneously to reduce the consumption of fluoride and nitrate ions in the pickling process by the order of 80% or more and to recover the major part of the zirconium ions which have been dissolved. The process makes it possible largely to replace the consumption of nitric and hydrofluoric acids by the consumption of less expensive sulfuric and hydrochloric acids. Finally, the process makes it possible to provide a neat solution to a delicate problem of treating effluents to prevent pollution.
3. Summary of the Invention
The present invention concerns the treatment of effluents from acid pickling of half-finished and finished products of zirconium or zirconium-based alloys in order to recover the zirconium contained in the effluents and to recycle residual anions contained therein. More precisely, the process which is the subject of this invention provides for selective extraction of zirconium from the pickling solutions and recycling of the major part of the fluoride and nitrate ions contained in such solutions, which provides for a saving in cost and considerably reduces the dangers of pollution by the discharge of spent acid solutions. The process is applied in particular to treatment of solutions for pickling zirconium and zirconium alloys which are used as structural or sheathing materials in nuclear reactors. The process then makes it possible to recover the zirconium, generally in the form of an oxide of nuclear purity, which can be re-used directly in the cycle for producing the metal.
In a typical practice of the process which is the subject of the present invention, the used or spent nitrohydrofluoric pickling solution containing zirconium and possibly other metals in a dissolved state is passed through a column containing a suitably selected anionic ion-exchange resin which has been previously subjected to a suitable treatment. It is then found that, under given operating conditions, it is possible at the outlet from this column to achieve separation of zirconium from the fluoride and nitrate ions contained in the solution. Hydrofluoric acid is regenerated from the fluoride ions which were combined with the zirconium in the spent solution while zirconium is recovered initially in the form of a sulfate solution.